The present invention relates to an overcurrent breaking circuit for a direct current power supply. The present invention relates particularly to improvements in an overcurrent breaking circuit using an intelligent power switch (hereinafter referred to as "IPS" in this specification). While the present invention has been developed for use in a power supply circuit in an automotive vehicle, the present invention is not limited to automotive vehicles but may be widely utilized in other applications.
The IPS has been widely used in power supply circuits for electric devices in automotive vehicles. The IPS is an electric component which contains, in a single package, a semiconductor switch disposed in a main current path, a current detector circuit for detecting a current through the main current path, and a logic circuit for automatically switching off the semiconductor switch when an overcurrent is detected in an detected output of the current detector circuit. A prevalently used IPS has a rated current set at several amperes and an overcurrent breaking value set at several tens of amperes. The IPS has a function of detecting by itself an overcurrent in the main current path to break the main current path in response to its detection output. The built-in logic circuit is provided with an output terminal and an input terminal so that more complicated control may be performed utilizing the output terminal and the input terminal. More specifically, a CPU (microprocessor) may be externally provided separately from the IPS such that a signal from the output terminal is fetched into the CPU and a control signal is fed from the CPU to the input terminal to perform a variety of high level controls as well as to break the main current path in the event of the detection of an overcurrent.
As an example of the prior art, Japanese Patent Unexamined Publication No. Hei. 1-301432 describes a related technique. This technique, which is used in a control switch for an electric device for an automotive vehicle, fetches an output signal from a logic circuit into a CPU which processes the signal and supplies the processed signal to the logic circuit as an input signal to control the level of a detected overcurrent at two steps. Specifically, when a current is initially supplied to electric devices through manipulation from the driver's seat for starting an automotive vehicle in an inoperative state, a current may be detected temporarily in excess of a set overcurrent level due to a rotating machine and so on included in the electric devices which may cause a large rush current. However, since the rush current is a normal phenomenon which occurs for a short time period immediately after the current is conducted, and the current value on the main current path is converged to a steady value after the short time period has passed when the apparatus is operating normally. Thus, the control is performed such that the set overcurrent level is temporarily made higher for the short time period in which the rush current may be generated, and the set overcurrent level is changed to a lower value corresponding to a steady current after the rush current is converged. This prevents the main current path from being uselessly broken due to the detection of an overcurrent and simultaneously prevents an abnormal phenomenon from being passed by due to a uniformly set high overcurrent level.
The assigner of the present application and others have already manufactured and commercialized unit components or automotive vehicles employing a circuit which operates according to different principles from the circuit described in the foregoing document but has a similar object. More specifically, as illustrated in FIG. 1, an IPS is used on a main current path for supplying a direct current to electric devices to turn on an off a current therethrough, and an output signal from a logic circuit of the IPS is fetched into a CPU which applies a control signal processed thereby to the logic circuit as an input signal. The CPU is provided not only for controlling the IPS but also for controlling electrically controlled switches for an automotive vehicle, not shown in the figure, in a complicated and associated manner. In this circuit, a control is performed such that during a time period in which a rush current may be generated due to an electric device, a semiconductor switch internal to the IPS is repetitively turned on and off at a high rate to prevent a current flowing through a main current path from substantially exceeding a set overcurrent level.
The overcurrent breaking circuit shown as a prior art example as described above has experienced the following problem. Some electric devices supplied with a current through the IPS have a large capacitive load. A specific example of such electric devices is a compressor motor for a freezer installed in a freezer car. While a compressor motor per se is not a capacitive load, a noise preventive capacitor having a large capacitance is connected to a power supply circuit to prevent noise from introducing into a radio receiver and a computer unit installed in the same car through the power supply circuit to adversely affect these components. In addition, the compressor motor for a freezer is independently turned on and off in response to a detected temperature within the freezing room, in principle, irrespective of the power supply for the respective components which is turned on and off in association with the running of the car.
Thus, even if protective measures are taken so as not to execute emergency processing in response to a detected current exceeding an overcurrent level particularly set for a rush current possibly generated immediately after a current is supplied to electric devices when an automotive vehicle is started, as described in connection with the prior art example, since a freezer relies on its own temperature control to automatically turn on and automatically turn off during a We steady running of the vehicle, the freezer turning on causes a large current to flow for a short time period, resulting in detecting a current exceeding the overcurrent level which is set to monitor a steady state. Since this is a normal phenomenon, it is not necessary to execute operations for breaking the main current path or for generating an alarm, by the assumption that a fault has occurred, from the current exceeding the overcurrent level.